Fine hairline cracks in bisque ware

Just another note - the pots are not being taken out of the kiln too early, so it is not thermal shock either! JM

Where are the cracks, and what shape pots are you making?

Wow thanks for the quick response!

Well I have made both a teapot, a tagine, and a greecian urn. All completely different shapes, but all have the same characteristic cracking.

The tagine has on both lid and base. Teapot has on the sides and bottom. The urn has not been affected on the base, but appear on the sides and neck. JM

any chance your clay has been contaminated with another clay? when i do glass and mix some from different manufacturers, different coe can cause random cracks. or doesn't clay do that?

Charles,

Well, I cant say about the clay at college. It IS possible that the clay has been contaminated there. To be honest the way they reclaim clay isn't in the best controlled circumstances, and I have often found bits of metal in the clay that I have been throwing (v dangerous - and nearly slashed my hand open once). I do check the clay thoroughly for foreign objects now!

As for the teapot, I threw this approx 2 - 3 months ago, and has sat on the shelf whilst other items got priority. I cannot remember whether I used my own clay, or the colleges.

So I suppose that contamination may be the key, but I have not seen this in other students' work.

I leave my mind open.....

Thanks for the response.

JM

I had a similar problem many years ago using a clay with a high Staffordshire fireclay content. I was bisque firing at about 950oC and getting very fine cracks in the ware which opened up in the stoneware glaze firing. Turned out that at that temperature bisque the clay could not withstand the shock of its own cooling. The answer was either to bisque at 1000oC or soak the 950 bisque for about 35 minutes. I chose the former and it cured the problem.

I notice you are using a UK ISP so this might have some relevance to you.

What clay are you using and how high the bisque?

Steve Bath UK

In article , JM writes

sounds like a bad batch of clay? maybe your supplier forgot to add a key ingrediant? or maybe the stuff was too new to use?

steve

steve graber

Thanks Eddie, I really appreciate all your inputs everyone... I dont think that the conditions at college would get that bad - I think Health and Safety might have something to say about that. Plus, my studio at home is constantly heated - ok its not needed during the summer months, but as soon as it starts getting colder I put the heating on in there. It never gets under 10oC. Any other suggestions? JM

I dont know what stoneware clay they use at the college, but it is dark grey in colour and fires to a v light grey. They bisque fire to 1100oC (which I think is too high - personally).

The clay I use at home, is Potclays 1143M Draycott White and fire to 900oC - I have not had any problems in the past firing at this temp.

Mmmm - I could try firing to 1000oc I suppose, and see what happens! But it doesnt explain why the college are getting the same results - esp when they fire at 1100oC.

Food for thought. Thanks

Perhaps...cant say for the college though, as I dont know what clay they use. Probably bog standard school clay. JM

Just a thought, but try skipping the hot air gun on a few pieces. I'd guess that drying the surface much faster than the interior must be building up some stress.

If this turns out to be the problem, then the next question is how other folks get away with it. Different bodies? Or maybe some technique difference with the hot air gun? It might even be that a torch would work better than the gun by drying only a very thin layer very quickly, not penetrating quite so deeply.

Just a thought....

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Yeh, I constantly turn the pot so that the temp is even. I also keep the gun away some distance, so the heat is not fierce. My last tutor used to bring a large gas bottle and a blow torch to class, and we used to use that. Those who were not used to using the blow torch method would find that, if you were not careful, small chips would shoot off the ware! It was quicker, but I found my little gun perfectly adequate, and in my mind, a lot gentler on the ware. JM

As I said before, I dont know what type of clay the college uses, but my clay is 1143M Draycott white stoneware clay. Its about the whitest firing stoneware clay that Potclays sell.

JM

Hi Ken, I will try this, but the reason I use the hot gun is because the class is only 2hr duration.

If when leaving the items in the college wet cupboards, you return to find the cupboards were too wet and your hard work has been reduced to a soggy mess, or they are too dry and has completely dried out leaving it too hard to continue work on the following week.

So, because of this, what I always aim to do is complete whatever I start that session - hence using the hot gun.

Also, I sometimes take the project home to finish off there. I hate leaving items at the college as they are on constant display to all, often being moved around, and sometimes we return to find broken pieces. The classrooms are in constant use (by schools also), and we cannot always rely on the college staff firing greenware on a regular basis. Sometimes items are left for a couple of weeks in the kiln room waiting to be fired. I feel sorry for those who dont have the ability to finish their items at home, like I can.

However, I have done a test bowl and left it to dry there. I will see if the results are any different.

If its sooo bad there then why do I attend the college? - Well I like the company, its great to share ideas, and gain inspiration from others. Being stuck in my own studio can be lonely, and can easily get stuck in a rutt of making the same things over and over again. Its great to mix, have a laugh in the process, try new ideas once in a while, and if you do get a problem - well a problem shared, is a problem halved! But in this case, everyone is baffled!

Perhaps too fast drying is the key here, but my ex-tutor used a blow torch which was far more powerful (on the very same clay) and we had no problems then.

I'll let you know how the bowl turns out!

Keep the ideas flowing!

JM